The present invention relates to a method of treating vermiculite ore to lower the expansion temperature of the ore and improve the characteristics of the expanded vermiculite. The invention is further directed to intumescent sheet and other intumescent materials containing low temperature expandable vermiculite prepared according to the invention.
Vermiculite is a micaceous mineral, chemically identified as a hydrated magnesium-aluminum-iron silicate and characterized by a layered structure which exfoliates or expands in one dimension when heated at high temperatures or subjected to various chemical treatments, so as to be increased to many times its original size. The mineral ore occurs naturally in an unexpanded state, and is mined in conventional manner.
Since vermiculite as it is mined in the form of vermiculite ore is associated with other minerals as impurities, the crude vermiculite ore, after being reduced to particle size, has generally been beneficiated by various concentrating methods well known in the art wherein the gangue material is separated from the vermiculite particles as much as possible, the vermiculite screened into a number of component sizes and the vermiculite heated at elevated temperatures, typically approximately 1800.degree. F., until expansion occurs.
It has been an important goal to find a means and method of lowering the expansion temperature of vermiculite ore, both for economic reasons, and so that the vermiculite may be used in applications where a material is required which may be expanded in situ at relatively low temperatures. However, at lower expansion temperatures untreated vermiculite either will not expand at all, or will not yield an acceptable volume of expanded product.
It is known to increase the volume yield of vermiculite, and slightly decrease the expansion temperature, by osmotically swelling the vermiculite ore prior to expansion by treating the ore with various solutions containing alkali or alkaline earth salts. Such a method is described in U.S. Pat. No. 3,062,753. This method succeeds in increasing the yield of the ore, but the expansion temperature is still uneconomical and too high for the vermiculite to be used in many applications.
This method, and other previously known methods of treating vermiculite with salt solutions, also have several accompanying disadvantages. First, intumescent materials containing the expandable vermiculite thus produced tend to shrink upon initial heating, especially when the vermiculite is incorporated in intumescent gasket materials and expanded in situ under load, (this shrinkage is generally referred to in the art, and will be referred to herein, as "negative expansion"). This negative expansion may be deleterious in some applications, e.g. intumescent gasketing applications where it may interfere with the formation of a tight seal. Secondly, the disposal of the solutions used must be carefully monitored, especially when the salt used is an alkali or alkaline earth chloride, as these solutions are considered potentially hazardous to the environment. Furthermore, the residual chloride ions contained in the expanded vermiculite may be corrosive in certain applications and are undesirable in agricultural and horticultural applications.
The expansion temperature of vermiculite ore may be lowered, and the tendency of the sheet containing the vermiculite to undergo negative expansion reduced, by treating the vermiculite ore with ammonium cations, as disclosed in U.S. Pat. No. 4,305,992, the disclosure of which is incorporated herein by reference. However, the ammonium solutions used in this process tend to be relatively costly, and the disposal of the resulting waste stream also must be closely monitored as the waste is potentially hazardous. Also, the expansion temperatures thus obtained are still too high for many applications in which it would be desirable to use vermiculite.
It has been desired to use vermiculite in materials which are designed to expand when exposed to heat, i.e. intumescent materials. These intumescent materials are used in many applications including fireproofing, insulation and gasketing. Vermiculite is also useful in fireproofing materials referred to as endothermic materials, which block heat penetration by absorbing energy chemically. Intumescent sheet materials containing vermiculite generally comprise vermiculite, inorganic and organic fibers, low density fillers, e.g. hollow glass microspheres, and organic binders. Vermiculite is useful in these applications both because it expands when exposed to heat, typically from 8 to 20 times, and thus can seal flames and heat out of openings, ducts, and other areas, and because of its ability to block heat penetration by absorbing energy via chemically bound interlayer water and other volatiles.
An intumescent sheet comprising unexpanded ammonium ion exchanged vermiculite, inorganic fibrous materials and binders is disclosed in the aforementioned U.S. Pat. No. 4,305,992. This flexible intumescent sheet material is utilized in automobile exhaust catalytic converters as a mounting material by expansion in situ. The expanded sheet then holds the ceramic core or catalyst support in place in the container or canister. The thermal stability and resilience of the sheet after exfoliation compensate for the difference in thermal expansion of the metal canister and the ceramic substrate, for vibration transmitted to the fragile device and for irregularities in the metallic or ceramic surfaces. Generally, in this application intumescent sheets are expanded in situ by the catalytic converter manufacturer by connecting the converter to an automobile exhaust system and running the automobile until the intumescent sheet expands fully.
One disadvantage of conventional intumescent materials is that they generally undergo negative expansion when exposed to temperatures of from 230.degree. F. to 750.degree. F. In gasket applications, such as the packing and mounting of automobile catalystic converters, this negative expansion may cause the mounted catalyst support to become undesirably loose when these temperatures are encountered and remain loose until such time as the intumescent sheet material has passed through the negative expansion region and expanded sufficiently to recover its original thickness.